The present invention relates to a communication system, and more particularly, to a method and calibration system of IQ DC offset and imbalance calibration for a communication system by utilizing analytic formulas.
In conventional radio frequency circuits, some non-ideal effects of the transmission signal are induced by a mismatch of the circuit elements. Please refer to FIG. 1. FIG. 1 is a diagram of a conventional receiver 100. The receiver 100 comprises an antenna 110, a low noise amplifier (LNA) 120, two mixers 130 and 140, two low pass filters (LPFs) 150 and 160, and two analog-to-digital converters (ADCs) 170 and 180. The antenna 110 receives a radio frequency (RF) signal, and the LNA 120 amplifies the RF signal. The mixer 130 generates an analog signal Sa1 by mixing the RF signal and a first carrier COSωct while the other mixer 140 generates an analog signal Sa2 by mixing the RF signal and a second carrier SIN(ωct+ψ). The LPFs 150 and 160 remove the high-frequency components of the incoming analog signals Sa1 and Sa2. Additionally, the ADCs 170 and 180 respectively convert the analog signals Sa1 and Sa2 into the corresponding digital signals Sd1 and Sd2 for subsequent signal processing.
The phase difference between the ideal first carrier COSωct and the ideal second carrier SIN(ωct) is 90°. Given an ideal phase difference, the analog signals Sa1 and Sa2 are orthogonal. The analog signals Sa1 and Sa2 are commonly called In-phase signal and Quadrature-phase signal, respectively. However, due to the drift of temperature, process variation, and other factors, the phase difference between the actual first carrier and the actual second carrier may not be exactly 90°. A phase offset ψ between the first carrier COSωct and the second carrier SIN(ωct+ψ) indicates this possible effect. The phase offset ψ between two carriers may cause the in-phase signal Sa1 and the quadrature-phase signal Sa2 to be non-orthogonal. The case of this non-orthogonal relationship is called IQ mismatch. Generally speaking, the IQ mismatch includes two components: gain mismatch and phase mismatch. The phenomena of IQ mismatch may degrade the performance of the bit error rate (BER) of the communication system. Thus, it is necessary to calibrate IQ mismatch to improve the performance of the communication system and to increase the bit rate of the communication system.
In addition, the receiver 100 may have carrier leakage problem. Carrier leakage occurs when the input is connected to ground, but the mixers 130, 140 still produce output signals. Carrier leakage is usually present at the output of the mixers 130, 140. Non-zero voltage of the mixers 130, 140, the coupled signal coming from a local oscillator, and the mismatch of mixers 130, 140, all may cause the problem of carrier leakage.
In the related art, additional compensation circuits are implemented to compensate for the non-ideal phenomena in the baseband circuit. For instance, the related compensation scheme repeatedly calibrates the compensation values utilized by the compensation circuits, and then measures the results to determine the best compensation values that are capable of minimizing the effects attributed to IQ mismatch. The related art scheme is called a binary search method. The desired compensation time is related to the range of available compensation values and accuracy. However, significant time is required to seek and find the optimum compensation values by this related art scheme. Therefore, the related art scheme is not applicable to being utilized in portable communication devices, such as cellular phones.